SAW devices have been widely used for filtering applications in many telecommunication systems. The SAW filter is generally composed of several resonators. Further, there are several SAW resonator configurations that are well known in the art. One configuration for a SAW resonator comprises an interdigital transducer (IDT) embedded between the two reflectors. The SAW resonator is typically fabricated on a high coupling piezoelectric substrate such as Lithium Tantalate (LT) or Lithium Niobate (LN), by way of example. The IDT generally comprises interdigitated metal electrodes connected to opposing busbars. When a radio frequency (RF) field is applied to the opposing electrodes, the piezoelectric material will convert electrical energy to mechanical energy in the form of acoustic waves. These acoustic waves are reflected by the outer reflectors thereby forming a standing wave resonator. A plurality of resonators is typically arranged in series and parallel arms to realize a ladder filter network. Another configuration of a SAW filter comprises a coupled resonator in which multiple interdigital transducers are embedded between the reflectors.
It is well known in the art that the use of oxide layers or coatings, such as a silicon dioxide (SiO2) layer formed above the transducer provides an improvement in the temperature characteristics of the SAW filter performance as the linear coefficient of expansion of the silicon dioxide layer is opposite the acoustic temperature coefficient for that of LT or LN piezoelectric substrates. By way of example, U.S. Pat. No. 7,209,018 to Nakao et al. discloses a SAW filter in which the SiO2 layer that covers the electrodes has convex and concave portions. The electrode metal thickness is in the range of 1%-2.5% of the wavelength for improved insertion loss and temperature characteristics.
Further, U.S. Pat. No. 7,230,365 to Nishiyama et al. discloses that the lack of planarization actually degrades the insertion loss of the SAW resonator. It is well known in the art that once an oxide layer is put onto the SAW transducer pattern, a planarization process should help to improve loss degradation. To improve the filter performance, Nishiyama teaches the use of two SiO2 layers. The first layer of SiO2 is disposed between the metal electrodes on the piezoelectric substrate, the thickness of the layer substantially equal to the thickness of the metal electrodes. The second layer of SiO2 provides a cover to the first oxide layer and the metal electrodes. Nishiyama emphasizes that the devices require the first SiO2 layer to be of the same thickness of that of the electrode metal to provide improved insertion loss.
In addition, the quality factor of the SAW resonators under a silicon dioxide layer is greatly affected by the integrity of the oxide material and by the flatness of the top surface of the oxide layer. The coupling coefficient and the temperature coefficient of frequency (TCF) of the buried SAW resonator is determined by the oxide layer thickness over the resonator. It is observed that the double layers of SiO2 as described by Nishiyama consistently result in “voids” in the vicinity of the electrode edges. These voids, by way of example, are portions or traces of SiO2 that exhibit lower density when compared to the overall layer. Based on the teachings of the present invention, voids or seams within the oxide layer can significantly degrade the quality factor of the resonator. The present invention provides devices and methods for the deposition of oxide layers resulting in a produced device having a significant void reduction, thus providing an improved insertion loss and quality factor of the resonator.